This Invention relates to recovery of oil and gas from wells, and more particularly to breaking fluids inside formation pores when using viscoelastic surfactant based fluid systems (VES's) as carrier fluids and treatment fluids.
There are many applications in which breakers are needed to decrease the viscosity of treatment fluids, such as fracturing, gravel packing, and acidizing fluids, that have been viscosified with polymers, with crosslinked polymers, or with viscoelastic surfactants. Most commonly, these breakers act in fluids that are in gravel packs or fractures; some breakers can work in fluids in formation pores. Breakers decrease viscosity by degrading polymers or crosslinks when the viscosifiers are polymers or crosslinked polymers. Breakers decrease viscosity by degrading surfactants or destroying micelles when viscosifiers are viscoelastic surfactant fluid systems. Most breakers are solids, for example granules or encapsulated materials that do not enter the formation.
There is sometimes a need to break viscous fluids within the pores of formations, for example when viscous fluids enter formations during fracturing, gravel packing, acidizing, matrix dissolution, lost circulation treatments, scale squeezes, and the like. Breakers that are effective inside formations will be called internal breakers here. These fluids that enter the formation may be main treatment fluids (such as fracturing fluids) or they may be secondary fluids (such as flushes or diversion fluids such as viscoelastic diverting acids). Typically it is necessary that the break be delayed, that is that the breaker not act until after the fluid has performed its function.
Compositions and treatment methods using a delayed internal breaker that does not affect the initial properties of the fluid and acts without mechanical or chemical action by the operator, would be of value. It would be desirable to have a number of such materials so that they could be used under different subterranean conditions, for example different temperatures and different formation fluid chemistries.